I will repost some bits of my posts from the past.
One is obliged to confess all mortal sins in number and kind (and circumstance that changes the kind -- like it was your brother you murdered! or the gold cup you stole was from the Church (hence sacrilege))
Venial sins are recommended to be confessed. They can also be forgiven in many other ways.
Of course after a good examination we may not ...remember everything..that is ok...we do what we can..God forgives the forgotten ones too (we need of course to be sorry for all our mortal sins...and amended)...then if we later remember one we forgot..we are to confess it in the next confession after we remember it. (this is different from if we intentionally hide a mortal sin...)
If something is doubtful one notes it is doubtful in some way. While one is not strictly obliged to confess doubtful mortal sins -- it is normally recommended to (though those with scruples may be recommended not to).
Also we may not know the numbers ...we do what can ....and if we do not know..we make an approximation according to our knowledge from our examinination ...3-5x, 5-10 a month for the last 20 years, somewhere around 50 times, around 10x, or it was several times definately more than 10 ....it may be that one has to say: "a few times", "several times" "many times" "many many times" etc
Of course one needs contrition and a firm purpose of amendment.
[Honestly forgetting to confess mortal sins
-is well --forgetting to do so. Hiding such is a different story. Assuming one was contrite and amended and intending to confess all mortal sins --and just forgot to say some ---
they are absolved indirectly.
One is to confess then in the next confession. Of course ones memory could honestly slip again despite ones intent etc to remember.
(Now I will note for some out there -- who struggle with scruples -- they may be in a different boat in some cases. For some seek to confess all sorts of un-needful things --they need a regular confessor who can guide them)
http://jimmyakin.com/2007/03/specific_confes.html (Jimmy Akin of Catholic Answers).
As to forgotten mortal sins: jimmyakin.com/2006/09/a_reader_writes_1.html
As to "doubtful mortal sins" (like I am doubtful if I gave complete consent to it) the general recommendation is that those of an ordinary conscience should confess them (noting they are doubtful) and especially those of a lax conscience (again noting the doubt). Though there is not a strict obligation to confess them.
Those who struggle with scruples are often rather recommended to not confess them directly (if they do they too would not it is doubtful).
Jesus is the Good Shepherd ..he loves you --in him is* true life *
(and thus in confession is true life!)